Uploading the translation
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Navigate to the > Administration > Overview > Look & Feel > UI Languages page.
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Click the button Customize in the Your custom languages area for the language into which you want to translate.
The Your language dialog box opens.
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Upload the translated Excel file:
Drag and drop the file to the bottom area of the dialog box.
Click on the bottom area and select the Excel file.
OR
The file is uploaded.
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Close the dialog by clicking ✕ in the top right.
You have uploaded the translation.

In the next - and final - step, enable the customer-specific language, see Enable Customer-Specific Language.